The present invention relates to a fall detection device with a three-axial acceleration sensor for detecting a fall.
Recently, a dimension of a hard disk device has been reduced while a capacity thereof has been increased. Accordingly, various types of mobile devices such as a cellular phone are provided with hard disk devices. When a mobile device is provided with a hard disk device having relatively low impact resistance, it is necessary to protect information stored in the hard disk device from an impact of a fall, so that the information is prevented from being damaged. To this end, it has been tried to detect a fall of a mobile device, so that a magnetic head is retracted from a magnetic disk of a hard disk device.
A conventional fall detection device is provided with a three-axial acceleration sensor for detecting acceleration components in three axes crossing perpendicularly with each other. When each of the acceleration components in the three axes detected by the three-axial acceleration sensor changes from a static state and all of the acceleration components become below a threshold value (one third of gravitational acceleration), it is determined that an object is under a free fall state (refer to Patent Reference 1).
In the free fall state, the three-axial acceleration sensor is under a zero gravity state, and the acceleration components in three axes become zero. When an object having a fall detection device falls while rotating (rotating fall), an acceleration component in one axis away from a rotational axis is detected due to a centrifugal force accompanied with the rotation. When the acceleration component does not become below the threshold value, it is difficult to accurately detect a fall even though the object is actually falling. Accordingly, information stored in a hard disk device in a mobile device may be damaged, thereby lowering reliability of fall detection.
In order to detect the rotating fall, a conventional fall detection device is provided with a pair of crossing electrodes arranged on an X-axis and a Y-axis, and a weight is disposed at an origin of the X-axis and the Y-axis. Then, while rotating, alternate current signals having phases inverted with each other are applied to a pair of the crossing electrodes on the X-axis, respectively. Further, alternate current signals having phases inverted with each other and shifted by 90 degrees from those of the alternate current signals applied to the X-axis are applied to a pair of the crossing electrodes on the Y-axis, respectively.
A pair of the crossing electrodes on the Y-axis detects mechanical deformation due to Coriolis force in the Y-axis direction at a node of the alternate current signals applied to the X-axis, so that angular velocities around the Y-axis and the Z-axis are detected. A pair of the crossing electrodes on the X-axis detects mechanical deformation due to Coriolis force in the X-axis direction at a node of the alternate current signals applied to the Y-axis, so that angular velocities around the X-axis and the Z-axis are detected.
Further, a voltage detection device detects a voltage component overlapped on the alternate current signals applied to each of the axes, so that an acceleration component in each of the axes is detected. Accordingly, it is possible to detect the angular velocity around each of the axes and the acceleration component in each of the axes (refer to Patent Reference 2).
Patent Reference 1: Japanese Patent Publication No. 2005-37300
Patent Reference 2: Japanese Patent Publication No. 2000-162235
In Patent Reference 2, while rotating, the specific alternate current signals are applied to pairs of the crossing electrodes on the X-axis and the Y-axis, respectively. The angular velocities are detected from the outputs of the X-axis and the Y-axis at the nodes of the alternate current signals applied to the X-axis and the Y-axis. The voltage detection device detects the voltage components overlapped on the detected alternate current signals applied to each of the axes, so that the acceleration component in each of the axes is detected. Accordingly, it is necessary to provide a supply source of the alternate current signals and the voltage detection device in the fall detection device, thereby making a structure of the fall detection device complex and increasing a size of the fall detection device.
A mobile device may rotate in a situation other than the free fall. For example, when a user holds a mobile device in a hand and swings the mobile device (normal telephone operation, showing a display of the mobile device to others, etc.), the mobile device rotates associated with the swing movement. Accordingly, even though it is possible to detect an angular velocity and determine that the mobile device rotates, when it is erroneously determined that the mobile device is under a free fall state even though the mobile device rotates associated with the swing movement, a magnetic head is retracted too frequently. As a result, power consumption increases, and it takes long time to read and write information from and in the hard disk, thereby decreasing a processing speed of the mobile device.
In view of the problems described above, an object of the present invention is to provide a fall detection device with improved reliability having a small size.
Further objects and advantages of the invention will be apparent from the following description of the invention.